Aircraft: CRJ200
Phase of flight: Descent
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy

I was training on the East Final. We pulled in the other finals and were coming out of outboard widely spaced self monitored ILS approaches north to 36L and 36R and were going into a stagger. The CRJ was eastbound and descending out of 070 for 050 expecting 36L. The Dash 8 was westbound at 060 issued a descent to 040 expecting 36R. I told my trainee to slow and turn the CRJ. He slowed him to 210 KTS and did not turn. I told him again that's too fast you have to go all the way back and he's still going to need turned out. He slowed to 170 KTS but did not turn. As the CRJ got closer to the final I told him to change his landing runway to 36C so we could use reduced separation and that he was going to need to go across the localizer for spacing since he didn't turn him like I told him to. As the spacing became greater between the CRJ and the aircraft already on the final for 36R the CRJ needed turned in to the localizer. The Dash 8 was on opposing base a little further south than CRJ and still descending through 050. Instead of tuning the CRJ; the trainee turned the Dash 8 in toward the CRJ. I thought for a second that I missed Dash 8's altitude and that he must be a 040 if he was turning in but the data block of Dash 8 was overlapped by a VFR aircraft northbound at 035. I moved the data blocks to see the information and saw that Dash 8 was still not out of 050. I turned CRJ to heading 030 and then turned Dash 8 to a heading of 250 try to keep 3 miles. Shortly after; the CA went off. The *T function showed 3.8 miles and closing. I don't know how close they actually got before they passed. This trainee does not listen to what I tell him. I have told him countless times that you can not be anywhere near that 36R localizer unless you're a 040. I don't care if you have to turn back into the downwind to get down then you do it but don't be at anything other than 040 near the east localizer period. Ten minutes prior to this situation; he had the same thing going on with a different Dash 8. Descending westbound slowly and he told him to reduce speed to 170 as he was descending out of 052. I asked him what's more important his speed or his altitude? He said his altitude. So he then told the Dash 8 to expedite down to 040. That Dash 8 got down but only because I said something and the trainee didn't even bother to coordinate with the West Final to let him know he was still high. He seems to not believe what I tell him until the bad thing I was warning him about happens to him. Then he gets it. There has to be a certain level of trust between a trainer and trainee especially on the final where you're expected to put planes very close together and I no longer trust that he will do the right thing. He has a lot of hours on that position and should know better than to do what he did. Dash 8 had no business being turned in anyway since there wasn't even enough room for the CRJ to fit. Which is why he was going across the localizer in the first place and that would not have been needed if he would have listened to me and turned the CRJ out a little when he took the position. TRACON Controller providing OJT described a loss of separation event during simultaneous approach procedures claiming the student failed to take steps as directed resulting in the conflict.